How to force files saved to the system's Downloads folder to be stored on an external drive?

Solution 1:

You could do it with a Automator Folder Action workflow:

• Automator > New > Folder Action
  • Set: Folder Action receives files and folders added to Choose folder to: Downloads
• Add the following Actions:
• Get Folders Contents
  • [ ] Repeat for each subfolder found - Leave unchecked.
• Filter Finder Items
  • Find files where:
    • None of the following are true
    • Name ends with .download
      • This is to avoid moving a file Safari is currently downloading.
    • Kind is folder
      • This is to avoid moving just the contents of the folder.
    • Kind is other alias
• Move Finder Items
  • To: Downloads [ ] Replacing existing files
    • Obviously in this instance Downloads is the one on the external drive.
    • You can choose to check or not: Replacing existing files

• New Alias

  • In: Downloads
    • Obviously in this instance Downloads is the one in your Home folder.
      If you do not want an alias created for each moved file, then omit this action.

• Save the Automator Folder Action workflow, e.g.; Move to External Downloads

To stop and or remove a Folder Action: In Finder, you can select the target folder and right-click on it and select Folder Action Setup... from the Services menu section of the Context menu.